The present invention relates to gate turn-off thyristors (hereinafter referred to as GTO thyristors), and more particularly to a GTO thyristor suitable for a high speed operation.
A GTO thyristor switched (turned on/off) in accordance with a gating signal includes a semiconductor substrate which, in turn, having a pair of opposing main circular surfaces and four continuous pnpn-layers disposed between the main opposing surfaces. The pn-pn layers include an outer n-(emitter) layer and an intermediate p-(base) layer on a first one of the main surfaces and with a p-(emitter) layer and an intermediate n-(base) layer on a second one of the main surfaces. The n-emitter layer is divided into many strip-like areas disposed in a radial multi-concentric manner, while the p-emitter layer is divided into many areas such that when the n-emitter layer is projected onto the other main surface, the areas of the p-emitter layer overlap with the strip-like area of the n-emitter layer. Also included are main electrodes disposed on the cathode side of the substrate. The main electrodes are put in low resistance contact with respective areas of the n-emitter layer on the first main surface of the semiconductor substrate. An electrode is included on the anode side of the substrate and it is put in low resistance contact with the p-emitter and n-base layers on the other main surface of the semiconductor substrate. Control electrodes are put in low resistance contact with the p-base layer on the first main surface of the semiconductor substrate so as to surround the respective areas of the n-emitter. A cathode side electrode plate is electrically connected with the cathode side main electrodes. It has a diameter smaller than the semiconductor substrate and larger than the n-emitter layer. An anode side electrode plate is electrically connected with the anode side main electrode. It has a diameter smaller than the substrate and larger than the p-emitter layer. A control electrode lead is electrically connected with the control electrode at substantially center of the one main surface of the semiconductor substrate. (A GTO thyristor of this type is disclosed, for example, in JP-A-63-15464 (JP-B-6-1831)). The aforementioned GTO thyristor is enclosed within a package that includes a pair of block-like external electrodes and a cylindrical insulating ring, which connects the external electrodes with each other and which is supported between the external electrodes such that it is pressed by those external electrodes. The block-like external electrodes are smaller in diameter than the respective electrode plates so as to avoid stress concentration on the semiconductor substrate when the GTO thyristor is pressed. The external electrodes also function as a path which transmits heat generated in the semiconductor substrate to the outside of the package, for example, to cooling fins.